If you are swallowing some of your vape, you could get more queasiness or nausea, my husband gets that effect. I don't think LARGE amounts of vapor make it to your stomach but it's quite possible a small amount does. Because, during the inhale, (I'd imagine especially if you are using DL and going through more liquid, that means more nic will reach your stomach, and that may cause upset.
It could be the vehicle by which you are 'swallowing" sweetner, as well. I'd have a hard time thinking that sweetener (many of them are diuretics, they may give you some gastric discomfort) but if you are swallowing some of it, well, it may be enough to cause an effect, I'd think. Some sweeteners can be fairly potent laxatives, and that can upset your stomach.
Please note I am totally speculative here, I don't know what is going on, but those are the ideas that occurred to me. If it persists, I might see a doctor, for sure, to either help you isolate what is going on (it may or may not be vape related even ,we don't know) but you may be able to get help from you doc concerning how to perhaps isolate vape ingredients, one at a time.
This IS a time (if you get cleared by your doc) where DIY MAY be of use to you, as you'd have total control over your ingredients. It usually takes a while to get going and discover flavors, nicotine and recipes you enjoy, but you would have total control in the sense that you might start to learn what flavors (if it is flavors and not something else) irritate you vs .the ones you can cope with and different types of sweeteners and etc . Unfortunately, many flavor manufacturers have a bit of a "black box" surrounding them, and it's hard to know what is in them. I have a flavor I cannot vape, it's vanillas. IDK why, but they make me hoarse so I avoid them (I DIY).
Best of luck in getting it figured out.
Anna